Method for making a color image sensor with recessed contact apertures prior to thinning

ABSTRACT

The invention relates to method for making a color image sensor. The method comprises:
         the formation, on the front face of a semiconductive wafer ( 10 ), of a series of active zones (ZA) comprising image detection circuits and each corresponding to a respective image sensor, each active zone being surrounded by input/output pads ( 22 ),   the transfer of the wafer by its front face against the front face of a temporary supporting substrate ( 20 ),   the elimination of the major part of the thickness of the silicon wafer, leaving a fine silicon layer ( 30 ) on the substrate, this fine silicon layer comprising the image detection circuits.       

     Furthermore:
         firstly, layers of color filters ( 18 ) are deposited and then etched on the semiconductive layer thus thinned,   secondly, prior to the transfer of the semiconductive wafer to the substrate, on the front face of the wafer, metallized apertures ( 25 ) are formed extending to a greater depth than the elements of the image detection circuits formed on the surface of the wafer, and the step of elimination of the major part of the thickness of the semiconductive wafer includes the baring, from the rear, of the metallization ( 22 ) of the metallized apertures.

The invention relates to electronic image sensors, and especially tovery small-sized sensors with dimensions that enable the making ofminiature cameras such as those that are to be incorporated into aportable telephone.

Apart from great compactness, the image sensor should have highsensitivity under weak light and excellent colorimetrical performance.

Furthermore, the entire camera needs to be made by the most economicalmethods possible so as not to make the apparatus prohibitively costly.

To achieve this result, it is sought, firstly, to make the image sensorand the electronic processing circuits if possible on a same siliconsubstrate and, secondly, as far as possible, to carry out the depositionof the different layers, the etching operations, the heat-processingoperations etc. collectively on a silicon wafer comprising manyidentical sensors, and then dice the wafer into individual sensors.

However, the methods hitherto proposed for making color image sensorsand the structures of these sensors are not entirely satisfactory fromthis viewpoint. The methods of manufacture are not industriallyefficient; they remain far too costly and their efficiency is far toolow for large-scale manufacturing applications, or else the performanceof the image sensor is not high enough.

The present invention proposes a method of manufacture and acorresponding image sensor that minimizes the costs of manufacture whilepresenting excellent quality and especially compactness, highsensitivity and high colorimetrical performance.

To this end, the invention propose a method for making an image sensor,comprising:

-   -   the formation, on the front face of a semiconductive wafer, of a        series of active zones comprising image detection circuits and        each corresponding to a respective image sensor, each active        zone being surrounded by input/output pads,    -   the transfer of the wafer by its front face against the front        face of a temporary supporting substrate,    -   the elimination of the major part of the thickness of the        semiconductive wafer, leaving a very fine semiconductive layer        comprising the image detection circuits on the substrate,    -   this method being characterized in that:        -   firstly, layers of color filters are deposited and then            etched on the semiconductive layer thus thinned,        -   secondly, prior to the transfer of the semiconductive wafer            to the substrate, on the front face of the wafer, metallized            apertures are formed extending to a greater depth than the            elements of the image detection circuits formed on the            surface of the wafer, and the elimination of the major part            of the thickness of the semiconductive wafer includes the            baring, from the rear, of the metallization of the            metallized apertures,        -   and finally, the substrate is diced into individual sensors            after the deposition and the etching of the color filters.

Preferably, the active zones comprise a matrix of photosensitiveelements as well as control circuits of the matrix and associatedimage-processing circuits receiving signals coming from thephotosensitive elements of the active area. The circuits thus associatedwith the matrix are preferably masked against light by a layer ofaluminum, only the matrix being exposed to light.

The transfer of the semiconductive wafer to the temporary substrate canbe done by gluing, classic soldering, anodic bonding or by simplemolecular adhesion (i.e. through the very great force of contact betweentwo surfaces having great planeity).

The thinning of the semiconductive wafer after transfer to the substrateand before the deposition of the filters can be done in many differentways: thinning by lapping, chemical thinning, a combination of bothtypes of thinning (firstly mechanical thinning and then chemicalfinishing or else mechanical machining in the presence of chemicals).The thinning can also be done by a preliminary embrittlement of thewafer at the desired dicing level, in particular by in-depth hydrogenimplantation in the desired dicing plane. In this case, the hydrogenimplantation is done at a shallow depth in the semiconductive waferbefore the transfer of the wafer to the substrate. The thinning is thendone by heat processing which dissociates the wafer at the level of theimplanted dicing plane, leaving a thin semiconductive layer in contactwith the substrate.

The very great thinning of the wafer reduces its thickness from severalhundreds of micrometers before transfer to the substrate to 3 to 20micrometers after transfer to the substrate. Thinning is a major factorin the quality of the sensors since it enhances colorimetricalperformance and sensitivity. With non-thinned sensors, illuminated bythe side in which there are formed the numerous insulating andconductive layers that serve to define the image detection circuits, thelight that has crossed a color filter is scattered on photosensitivedots corresponding to different colors, thus impairing colorimetricalperformance. Furthermore, the sensitivity of a thinned sensor isimproved because the photons reach a wider silicon region than in thecase of the non-thinned sensors, since they are not stopped by the metallayers which are opaque and take up a large part of the surface areacorresponding to each photosensitive dot.

It will be understood that the thinning, however, complicates theproblems of manufacture because, after thinning, the silicon loses itsrigidity and becomes very brittle, and that, furthermore, there arisesthe problem of connecting the image detection circuits with theexterior. The solution of the invention mitigates this difficulty andenables the making of the image sensors with great efficiency.

The connection pads of the sensors thus made are in the front of thesubstrate, on the side where the thinned silicon is located, the lightbeing received from the same side for the formation of an image.

The substrate and the silicon layer are in close contact and the activecircuit elements of the wafer are therefore well protected on this side.

Finally, on the thinned silicon layer, covered with color filters, it ispossible to place either a transparent sheet or a passivation layer oragain microlenses facing each sensor. These operations are preferablycarried out on the substrate in wafer form, before it is diced intoindividual sensors.

For example, the thickness of the substrate is about 500 micrometers fora substrate with a diameter of 15 to 20 cm. The thickness of the siliconwafer is 500 to 1000 micrometers before thinning (with a diameter of 15to 30 centimeters), and then 3 to 20 micrometers after thinning.

Planarization layers, made of polyimide for example, may be deposited onthe silicon wafer before transfer to the intermediate substrate.

Other features and advantages of the invention shall appear from thefollowing detailed description, made with reference to the appendeddrawings, of which:

FIG. 1 shows the structure of an image sensor made on a silicon waferbefore the positioning of color filters;

FIG. 2 shows the formation on this wafer of apertures filled with ametallization layer;

FIG. 3 shows the operation of transfer of the silicon wafer by its frontface to a supporting substrate;

FIG. 4 shows the supporting substrate with the silicon wafer afterthinning of the wafer;

FIG. 5 shows the substrate, bearing a thinned silicon layer on which amosaic of color filters has been deposited.

FIG. 1 shows the general structure of a silicon wafer on which classictechniques have been used to make the image detection circuits of amultiplicity of image sensors.

The silicon wafer 10 has a thickness of several hundreds of micrometers,for a diameter of 150 to 300 millimeters.

The image detection circuits (the matrix of photosensitive dots,transistors and interconnections) are fabricated on one face of thesilicon wafer, which may be called the front face and is the upper facein FIG. 1. Fabrication implies, firstly, various operations of diffusionand implantation in the silicon, from the upper face of the wafer, toform especially photosensitive zones 12, and, secondly, successiveoperations for the deposition and etching of conductive layers 14 andinsulating layers 16 forming a stack on top of the photosensitive zones12. The insulating and conductive layers form part of the imagedetection circuits and enable the collection of electrical chargesgenerated in the photosensitive zones by an image projected on thesensor.

If the sensor were to be made by means of a classic technology, then amosaic of color filters would be deposited on the surface of the wafer.According to the invention, they are not deposited at this stage andpreliminary operations are performed.

For each individual image sensor formed on the silicon wafer,input/output pads surround an active surface comprising both a matrix ofphotosensitive zones and associated electronic circuits.

These input output pads (22 in FIG. 2) are connected to the conductivelayers 14 and, in the present invention, are formed as follows:apertures 25 are hollowed out in the stack of insulating layers 16 aswell as in the depth of the silicon substrate. The depth of theapertures 25 is greater than the depth of all the image detectioncircuits (photosensitive zones, interconnections, etc.) formed in thesilicon wafer. This depth corresponds more or less to the siliconthickness that will remain after the subsequent thinning of the siliconand will contain these image detection circuits.

The thinning of the silicon, which will be done by the rear of thewafer, will in principle reach exactly the bottom of the apertures 25.However, it will be seen that the depth of the apertures 25 may beslightly different from the desired thickness for the thinned silicon.Typically, the depth of the apertures 25 is about 5 to 20 microns insidethe silicon of the wafer, namely about 15 to 30 micrometers below thesurface of the stack of conductors and insulating layers 14, 16.

In the apertures 25, there is preferably formed firstly an insulatinglayer 26. Then a metal layer is deposited and etched. This metal layerwill form the connection pads 22. These connection pads come intocontact with one of the conductive layers 14. If, for this purpose, itis necessary to make apertures for the local baring of a layer 14, theinsulating layers 16 which cover the layer 14 are hollowed out locallybefore the deposition of the metal layer in the aperture 25.

No color filters are deposited at this stage but the wafer istransferred by its front face to a temporary substrate 20 (FIG. 3). Thesubstrate is a wafer having the same diameter as the wafer 10 and asimilar thickness to ensure the rigidity of the structure while it isbeing made. It may furthermore be constituted by another silicon wafer.The transfer can be done after the deposition of a planarization layerserving to fill the relief features created on the front face of thesilicon wafer by the operations of deposition and etching of the stackof conductive and insulating layers. This planarization layer does notneed to be transparent

FIG. 3 represents the structure on a smaller scale than that of FIG. 1in order to show the entire individual sensor comprising an active zoneZA and connection pads 22 around the active zone ZA.

The transfer of the silicon wafer to the supporting wafer 20 can be doneby several means. The simplest means could be quite simply that ofholding the wafer by molecular adhesion, since the great planeity of thesurfaces in contact generates very high contact forces. Gluing is alsopossible. It is also possible to carry out a soldering by means of theconnection pads 22 and corresponding pads formed beforehand in thesubstrate 20. In this case, it can furthermore be envisaged that thesubstrate 20 will comprise auxiliary active or passive circuit elements,that are connected to these pads and are therefore capable of beingdirectly connected to the image sensor.

After the silicon wafer has been transferred by the front face to thesupporting wafer, the major part of the thickness of the silicon wafer10 is eliminated so as to leave only a thickness of about 8 to 30micrometers, including the thickness of the stack of layers. Whatremains of the silicon wafer is no more than a superimposition of a fewmicrometers (for example about ten micrometers) for the stack of layers14, 16 and about three to 20 micrometers for the remaining siliconthickness, including the photosensitive areas 12. The remainingthickness is that of the layer 30 of FIG. 3 containing thephotosensitive zones 12 of FIG. 1.

The thinning reveals the metallization of the connection pads 22 so thatthey become electrically accessible by the rear face of the siliconwafer (the front face being the upper face in FIG. 1, pointing upwardsand covered with the substrate 20 in FIGS. 4 and 5).

If the thinning goes slightly beyond the deepest part of themetallization, it must allow a part of this metallization to remain inorder to make access possible. If the thinning is slightly short (by afew micrometers) of the deepest part of metallization, it is possible toenvisage the subsequent opening of access apertures through the rearface of the thinned silicon, these apertures baring the metallizations22.

The thinning operation can be done by mechanical machining (lapping)terminated by chemical machining, or by chemical machining only, or bymechanical machining in the presence of chemicals or again by aparticular method of separation necessitating a preliminary implantationof an embrittling impurity in the plane that will demarcate the thinnedsilicon layer.

In the case of this separation by implantation of impurities, theimplantation must be done before the transfer of the silicon wafer tothe supporting wafer. Indeed, the implantation is done by the front faceof the silicon wafer, throughout the surface of the wafer and at a depththat will define the dicing plane. The preliminary implantation ispreferably hydrogen implantation. It can be done at various stages ofthe making of the wafer, but the separation of the thickness of thewafer along the implanted dicing plane can be done only when the siliconwafer has been attached to the supporting wafer.

The upper surface of the thinned silicon layer 30 can be processed (finelapping, chemical cleaning, mechanical/chemical polishing, etc.) inorder to eliminate the surface defects, leading to a multiple-sensorwafer whose general structure is that of FIG. 2.

A mosaic of color filters 18 is then deposited on the surface of thelayer 30 (FIG. 4). However, one or more additional layers can bedeposited before the deposition of the color filters, especiallypassivation layers, anti-reflection layers and other layers, electricalactivation layers etc.

A glass film, or an individual lens for the image sensor, or a matrix ofmicrolenses having the same spacing pitch as the color filters 18 may bedeposited on the rear face (the face to be illuminated) of the structureafter the deposition and etching of the color filters.

The connection pads that have been bared by the thinning operation maybe used for a “wire-bonding” type connection (the wires 54 beingsoldered to the pads) or a “flip-chip” type of connection (the chipbeing placed upside down with the connection pads against thecorresponding pads of the printed circuit board with intermediateconductive bosses 56). In this case, the sensor is illuminated throughthe top of the printed circuit board and the board must have an aperturefacing the photosensitive matrix.

In these different embodiments, the structure formed on the substrate 40may be tested on the wafer by means of the connection pads. The test maybe performed in the presence of light, image patterns, etc.

The structure is diced into individual sensors for packaging only at theend of this fabrication process.

1. A method for making an image sensor, comprising the steps of:forming, on the front face of a semiconductive wafer, of a series ofactive zones comprising image detection circuits and each correspondingto a respective image sensor, each active zone being surrounded byinput/output pads; transferring of the wafer by its front face againstthe front face of a temporary supporting substrate; eliminating of themajor part of the thickness of the silicon wafer, leaving a fine siliconlayer on the substrate, this fine silicon layer comprising the imagedetection circuits, depositing, layers of color filters and then etchingon the semiconductive layer thus thinned, prior to the transfer of thesemiconductive wafer to the substrate, on the front face of the wafer,metallized apertures are formed extending to a greater depth than theelements of the image detection circuits formed on the surface of thewafer, eliminating the major part of the thickness of the semiconductivewafer includes the baring, from the rear, of the metallization of themetallized apertures, dicing, the substrate into individual sensorsafter the deposition and the etching of the color filters.
 2. The methodaccording to claim 1, wherein the remaining thickness of the thinnedsemiconductive layer is about 3 to 20 micrometers.
 3. The methodaccording to claim 1, wherein a sheet of transparent material is placedon the thinned semiconductive layer covered with color filters.
 4. Themethod according to claim 2, wherein a sheet of transparent material isplaced on the thinned semiconductive layer covered with color filters.